Power shift: North Korean leader Kim Jong-un (R) flanked by his uncle Jang Song-Thaek in 2012.

Reuters/Kyodo

North Korean leader Kim Jong-Un's uncle, seen as his nephew's political regent and one of the most powerful men in the country, has apparently been ousted and several associates executed, South Korea's spy agency says.

South Korea's National Intelligence Service (NIS) told a parliamentary committee that it believed Jang Song-Thaek had been removed as vice-chairman of the North's top military body, the National Defence Commission.

If confirmed, Mr Jang's ousting would mark the most significant purge at the top of North Korean leadership since Mr Kim succeeded his late father Kim Jong-Il in December 2011.

According to the NIS, Mr Jang was "recently ousted from his position and two of his close confidantes - Ri Yong-Ha and Jang Soo-Kil - were publicly executed in mid-November", lawmaker Jung Cheong-Rae told reporters.

North Korean military personnel were notified of the executions, Mr Jung said, adding that Mr Jang, 67, had since "disappeared".

The husband of Kim Jong-Il's powerful sister, Kim Kyong-Hui, Jang was seen as instrumental in cementing Kim Jong-Un's hold on power in the tricky transition period after his father's death.

He was often referred to as the unofficial number two in the hierarchy, with a number of analysts naming him as the real power behind the young Kim's throne.

"I can only guess that the roles played by Jang have caused some tension in the process of consolidating Kim Jong-Un's power," Seoul's Dongguk University professor Kim Yong-Hyun said.

"Jang once visited South Korea and witnessed many aspects of capitalist society, including the changes that have been happening in China.

"So he was the figure who was most likely to aggressively push for some reforms and opening of the North's system."

Uncle may have lost power to Kim Jong-Un confidant

Several analysts suggested Mr Jang may have lost out in a power struggle with Choe Ryong-Hae, a close Kim Jong-Un confidant who holds the military rank of vice-marshal and is director of the Korean People's Army's General Political Department.

In May, Mr Kim sent Mr Choe as his personal envoy to Beijing to hand deliver a letter to China's new president, Xi Jinping.

Mr Jang has fallen out of favour before. In 2004, he was understood to have undergone "re-education" as a steel mill labourer because of suspected corruption, but he made a comeback the following year.

He expanded his influence rapidly after Kim Jong-Il suffered a stroke in 2008 and he was appointed vice-chairman of the powerful National Defence Commission in 2010.

His wife Kim Kyong-Hui has also long been at the centre of power.

She was promoted to four-star general at the same time as Kim Jong-Un in 2010, a sign of her key role in the family's efforts to maintain its six-decade grip on power.

In the past year, she has been far less visible, with reports that she was seriously ill and had sought hospital treatment in Singapore on several occasions.

"This is the biggest political incident in North Korea since Kim Jong-Un took power," University of North Korean Studies professor Yang Moo-Jin said of Mr Jang's ousting.

Given Mr Jang's long, close ties to the ruling Kim family, analysts were divided over whether his dismissal would strengthen or weaken Kim Jong-Un's power base, with some suggesting the young leader may have been pressured into sacrificing his uncle.

Last month, the North Korean defector-run news website, Daily NK, known for its sources inside North Korea, published a long article about Mr Jang's diminishing political clout.

The Seoul-based website quoted multiple sources suggesting Mr Jang's influence in policy-making decisions had waned and that he appeared to have fallen out of favour with Mr Kim.